Aerosols for therapeutic purposes are generated and delivered to a desired location within a user's or patient's body with aerosol delivery devices. A fluid or liquid (i.e., medicament) to be aerosolised or nebulised is supplied to an aerosol generator of the aerosol delivery device, the fluid or liquid is aerosolised or nebulised by the aerosol generator and the resultant aerosol is supplied to the user or patient.
The fluid or liquid may be aerosolised or nebulised in the aerosol generator by a membrane with through holes. The fluid or liquid may be in contact with the membrane via gravitational force or a supply system. The fluid or liquid may be supplied via a supply system as e.g. a vibratable slide, vibratable plunger, vibratable wall and/or vibratable membrane.
The membrane may be a passive or an active membrane. In the case that the membrane is not vibrated by a vibrator, it is a passive membrane. The passive membrane may include a supply system and have e.g. a vibrator in contact with the fluid reservoir, wall, channel plunger, and/or supply system.
In the case when the membrane is vibrated by a vibrator, it is an active membrane.
An inhalation nebuliser of this passive membrane type is disclosed in U.S. Pat. No. 6,901,926 B2 as well as in US 2004/0045547 A1, which describe e.g. the nebulisers (inhalers) U1 and U22 from the company Omron. Further inhalation nebulisers of the passive membrane type are disclosed in US 121183747, WO 2006/094796 as well as in US 2009/0056708, which describe the Fox-POP, Medspray and Telemaq nebuliser technology. A further existing aerosol generator with a cantilever concept is disclosed in EP 0432992 A1 from the company Bespak.
An inhalation nebuliser of this active (vibrating) membrane type is disclosed in DE 199 53 317 C1, which describes e.g. the eFlow inhalation device from the company PARI. The aerosol membrane generator described in this document comprises a cylindrical liquid storage container which is delimited at one end face by a membrane having the shape of a circular disc. A liquid disposed in the liquid storage container contacts the side of the membrane facing the container.
DE 199 53 317 C1 further discloses an oscillation generator, for example, a piezocrystal, which surrounds the membrane in a circular manner and is connected thereto such that the membrane can be caused to oscillate by means of the oscillation generator and an electric drive circuit. The liquid abutting the membrane on the one side is conveyed through holes in the oscillating membrane to the other side of the membrane and is emitted on this side into a mixing chamber as an aerosol.
Known from the utility model DE 295 01 569 is an ultrasonic liquid nebuliser having a piezocrystal which is caused to oscillate electrically by an oscillator circuit, the oscillator circuit being supplied by a power supply device.
DE 295 01 569 describes an oscillator circuit which comprises a current limiting circuit and which is connected with an electronic temperature limiting circuit that compares a temperature-dependant electric signal occurring at the piezocrystal in a threshold circuit, the comparison signal of which activates a bistable circuit which blocks the oscillator when a limiting temperature in the piezocrystal is reached.
The disclosure of DE 295 01 569 is thereby directed at a protective mechanism for an ultrasonic liquid nebuliser in which the piezocrystal itself causes the liquid to oscillate and is in contact with a comparatively large amount of liquid. The liquid nebuliser described in DE 295 01 569 must furthermore accordingly use large currents in order to cause the large amount of liquid to oscillate.
Constant contact between the piezocrystal and the liquid is necessary owing to these large currents and the resulting large temperature differences in order to prevent destruction of the piezocrystal. If there is no longer any liquid present, the piezocrystal heats up very quickly and is destroyed if the oscillating circuit driving the nebuliser is not switched off immediately.
Only much smaller currents flow in inhalation nebulisers of the type described above, i.e. in inhalation nebulisers having membrane aerosol generators, and therefore only comparatively small temperature differences occur. In such inhalation nebulisers, the lack of liquid does not directly lead to heat-related damage to the piezoelectric elements. However, if a membrane inhalation nebuliser runs without a liquid (or fluid) load, this can, on rare occasions, cause the membrane to break.
However, it is also necessary in inhalation nebulisers having a membrane generator to reliably detect the presence of a liquid to be nebulised. This is because, on the one hand, the basis for a high dosage accuracy could be thereby created and, on the other hand, it is possible to reliably indicate the end of a therapy session to the patient. Normally it is desired to generate aerosol from the whole liquid to ensure the inhalation therapy success. Especially an early end of the therapy session, with remaining liquid in the reservoir, has to be avoided. In addition, when adherence to therapy is monitored with a patient in telemedicine applications, the signal provides assurance that the entire liquid volume and dose has been nebulised and delivered. Furthermore, by immediately disconnecting the inhalation therapy device, it is possible, for example, to save power, e.g. increasing the lifetime of a battery.
Therefore, the use of a protective mechanism such as described in DE 295 01 569 is not necessary in inhalation nebulisers of the type in question here and is not possible either owing to the much smaller currents and temperature alterations.
EP 1 558 315 A1 discloses an inhalation therapy device including a membrane aerosol generator. A detection device is provided for determining whether a liquid to be nebulised is available. Determination of whether liquid is present or not occurs in the detection device by comparing the detected value of an electrical parameter of the membrane aerosol generator with a value for this parameter stored in the detection device. For this purpose, the detection device may use empirically determined values for the detected electrical parameter or a value of the electrical parameter which was detected in a previous cycle. This determination process may be independently and separately performed at different measurement frequencies.
However, this approach of determining the presence of liquid in the membrane aerosol generator is sensitive to the structural details of the membrane aerosol generator, such as the thickness and bonding of the piezo-element, and susceptible to external influences, such as the surface tension and the temperature of the liquid to be nebulised and the pressure in the liquid reservoir. These factors can affect the determination accuracy, so that the presence or absence of liquid in the membrane aerosol generator may not be reliably identified.
Hence, there remains a need for an aerosol delivery device and an aerosol delivery method which allow for the presence of fluid or liquid to be aerosolised or nebulised to be reliably and efficiently detected.